Immunization with nucleic acids can induce both CD4+ and CD8+ mediated immune responses. Because both types of responses are important in conferring immunity to Toxoplasma gondii infections, immunization with DNA has the potential to induce protective immunity. To assess the feasibility of achieving immunity to T. gondii we performed vaccinations using a cDNA construct that contained the coding sequence for the mature peptide of the P30 surface antigen under control of the immediate early promoter of cytomegalovirus. Transfection of rhabdomyosarcoma cells with the plasmid led to the syn-thesis of a 30kDa protein immunoreactive with monoclonal antibody to P30. Sera of mice injected with the recombinant plasmid were positive against T. gondii by ELISA and immun-ofluorescence. P30-stimulated splenocytes from vaccinated mice produced primarily interferon-gamma. Analysis of the antibody isotype was consistent with a Th1-type response to the immunization. Upon challenge with 80 tissue cysts of ME 49 strain, all vaccinated mice survived, while 10 of 10 control mice died. When challenged with 20 cysts, the brains of vaccinated mice contained significantly fewer cysts at 6 weeks than did the brains of control mice. When vaccination was performed in rats, challenge with veg strain of T. gondii resulted in a significant reduction of cyst burden in the brain. These results suggest that nucleic acid vaccination may offer an effective means to obtain immunization against T. gondii.